Bivalves are regulators of coastal lagoons and provide a wide range of ecosystem services. However, coastal lagoons are sensitive to climate change. Our objective was to describe the drivers of the cascade of ecological events that occurred during a summer heatwave and which resulted in recruitment failure of the Pacific oyster Crassostrea gigas. Results show that elevated temperatures and salinity caused a shift in planktonic food availability toward smaller taxa. These trophic changes did not affect food accumulation by oyster larvae or their fatty acid composition but did affect post-metamorphosis success, with up to 24% fewer young metamorphosed postlarvae at some sites and no development of juveniles at all sites. This resulted in the failure of oyster recruitment and in the development of tubeworms, a trophic and spatial competitor that can better ingest small particles. This knowledge suggests that, in the context of marine heatwaves, the ecological limits of oyster larvae are narrower than their physiological limits.